1. Field of the Invention
This invention relates generally to fiber optic displacement sensors and particularly to a fiber optic gradient sensor for determining a direction and magnitude of an acoustic wave propagating through a fluid.
2. Discussion of the Prior Art
In marine seismic exploration, a plurality of pressure sensors are enclosed in a long tubular plastic streamer which may be several miles long and towed behind a ship. The earth layers beneath the ocean are insonofied by an acoustic source towed near the ship. The sonic waves generated by the acoustic source penetrate the earth and are reflected back to the ocean surface. The acoustic waves are detected by the sensors and are converted to electrical signals that are received and recorded by well known equipment aboard the ship.
The earth-reflected acoustic waves not only return directly to the sensors, but are reflected a second time at the ocean surface. The surface-reflected acoustic waves impinge upon the sensors a second time, delayed by a time proportional to twice the depth of the sensors. On the recording, the surface-reflected waves appear as secondary or "ghost" reflections of the earth's layers. Because the earth-reflected and surface-reflected waves arrive close together in time--a few milliseconds--they tend to interfere with each other, disturbing the first reflected signal.
Often, an object horizontally offset from the source, such as an offshore drilling rig or ship, reflects the acoustic signal back to the sensors at an angle other than vertical and interferes with the earth-reflected signals. It is therefore desirable to determine the propagation direction of the acoustic waves so that the earth-reflected signals may be distinguished from the surface-reflected and other secondary-reflected signals.
U.S. Pat. No. 3,952,281 issued to Parrack teaches a method of distinguishing earth-reflected signals from surface reflected and other secondary-reflected signals. Parrack's method employs two streamers towed behind a ship. One streamer is towed at a depth greater than the other streamer. When an earth-reflected signal passes both streamers, the uppermost streamer receives the signal by a time delay proportional to the vertical distance between the streamers. The delay time of the signal between the two streamers provides the vertical propagation direction. However, a major disadvantage to Parrack's method is the need for two streamers. Since each streamer may cost more than half a million dollars each, the two-streamer method tends to be cost-prohibitive. A second disadvantage stems from the fact that one streamer must be towed deeper than the other. This necessarily leads to slower towing speeds to prevent the deeper towed cable from strumming.
A two-cable system could be eliminated provided substantially compact sensors could be attained. If adequate sensors could be found, it would be possible to mount a substantially vertical array of conventional sensors within the same streamer. A major disadvantage to this system is that the streamer twists and turns as it is being towed behind the ship. Assuming that a vertical array of conventional sensors were mounted inside the streamer, the sensors would be incapable of determining which direction is up or down.
It is well-known that a pressure gradient exists between two vertical points in a column of fluid. If the pressure gradient could be measured between two vertically displaced sensors within the streamer, then the rotational position of the sensors could be determined and the surface-reflected waves could be distinguished from the earth-reflected waves.
Conventional marine pressure sensors are piezo-electric ceramic wafers. The wafers are generally mounted to operate in the bender mode. Transient pressure changes due to propagating acoustic/pressure waves, flex the piezo-electric wafers to generate an AC electrical charge. Piezo-electric wafers are also sensitive to static pressure changes. Piezo-electric wafers generate DC electrical charge when flexed in a single direction by changes in hydrostatic pressure. However, the DC current tends to leak off quickly through the associated circuitry. Therefore it is difficult to determine the hydrostatic pressure gradient surrounding the array.
A fiber-optic gradient sensor for use in a marine streamer is disclosed in U.S. Pat. No. 4,547,869 and is assigned to the assignee of this invention. The above cited gradient sensor consists of at least three optic-fiber coil pressure-sensors mounted at equiangular positions from each other and adjacent to the interior surface of the streamer housing. One end of each sensor coil is directionally coupled by a 3dB coupler to a common input fiber. The other end of the coil is directionally coupled in a similar manner to a common output fiber. Each sensor coil receives a coherent monochromatic pulse of light from a laser or LED through the input fiber. Transient and static pressures cause each coil to frequency modulate the light pulse passing therethrough. The modulated light pulse is then passed by the output fiber to a multiple input interferometer which then processes the data in a well known manner.
In the above system, the sensor coils must be placed adjacent the inner surface of the streamer skin to expose maximum coil area to the impinging pressure wave. This is necessary so as to apply transient and static pressure along the coil equally. This distribution is also necessary to obtain maximum distance between coils to provide a greater signal delay thereby to increase the sensitivity of the configuration for determining streamer orientation and acoustic wave propagation direction. The optic-fiber coils mounted adjacent to the inner surface of the streamer are susceptible to impact from sources outside the streamer which commonly damage the fibers.
It is an object of this invention to provide a substantially vertical planar array of sensors within a single streamer cable capable of detecting pressure gradients in a fluid.
It is another object of this invention to determine the orientation of a marine streamer rotating about its longitudinal axis so as to distinguish earth-reflected signals from surface reflected signals and from signals traveling substantially horizontally.
It is yet another object of this invention to determine direction and magnitude of acoustic waves propagating through a fluid utilizing phase modulated optical signals.
It is yet a further object of this invention to provide a more rugged gradient sensor less susceptible to damaging impacts.